1. Field of the Invention
The present invention relates to a self-emitting display driving circuit for generating gray-scale voltages according to display data and outputting them to a self-emitting panel such as an organic EL panel, and more particularly to a self-emitting display driving circuit for organic EL displays, etc., capable of adjusting a gamma characteristic (a gray-scale number vs. brightness characteristic).
2. Description of the Related Art
To display a high-quality image on an organic EL panel based on display data, it is necessary to set a gamma characteristic which matches the characteristics of the panel.
In the case of liquid crystal displays, Japanese Laid-Open Patent Publication No. 2002-366112 (Patent Document 1) discloses a circuit capable of adjusting the gamma characteristic of a liquid crystal display.
According to Patent Document 1, a gray-scale voltage generating circuit comprises a gamma adjustment control register made up of an amplitude adjustment register, a gradient adjustment register, and a fine adjustment register. The gray-scale voltage generating circuit also comprises: a ladder resistor for generating each (reference) gray-scale voltage from an externally supplied reference voltage with respect to ground GND, the ladder resistor being made up of variable resisters; a voltage divider circuit for further dividing each voltage generated by the ladder resistor (variable resistors); selector circuits for, according to the value set in the fine adjustment register, selecting some of the voltages generated by the voltage divider circuit; amplifier circuits for buffering the output voltages of the selector circuits; and an output ladder resistor for dividing the output voltages of the amplifier circuits into a desired number of gray-scale voltages. The resistance values of the lower side variable resistor and the upper side variable resistor respectively connected to the lower terminal and the upper terminal of the ladder resistor can be set by setting the amplitude adjustment register. The voltages generated by these two variable resistors are set to be the gray-scale voltages for the maximum and minimum gray-scale numbers, respectively.
Further, the resistance values of the two variable resistors respectively inserted at an upper middle position and a lower middle position of the ladder resistor can be set by setting the gradient adjustment register. The voltages generated by these two variable resistors are set to be the gray-scale voltages for gray-scale numbers which determine the gradient characteristic of the middle portion of the gray-scale number vs. gray-scale voltage characteristic curve.
Further, the gray-scale voltages generated by the above variable resistors (whose resistance values are set using the amplitude adjustment register and the gradient adjustment register) are subdivided by the voltage divider circuit to produce gray-scale voltages for fine adjustment. Then, some of the gray-scale voltages for fine adjustment are selected by the selector circuits according to the value of the fine adjustment register.
As described above, according to Patent Document 1, a liquid crystal display includes a gray-scale voltage generating circuit which adjusts each gray-scale voltage according to a desired gamma characteristic matching the characteristics of each liquid crystal panel by use of the amplitude adjustment register, the gradient adjustment register, and the fine adjustment register.
The prior art technique described in Patent Document 1 can be used to adjust the gamma characteristic of each of the R (red), G (green), and B (blue) color components in a liquid crystal panel, separately. However, each liquid crystal element in a panel exhibits the same characteristics, and therefore the above technique is intended to accommodate variations among the light transmittances of the R, G, and B color filters. In the case of organic EL panels, however, there are variations among the characteristics of the R, G, and B organic EL light-emitting element groups even in the same panel.
First, a description will be given of variations among the characteristics of self-emitting elements such as organic EL light-emitting elements with reference to FIG. 1. FIG. 1A shows I-B characteristics of a self-emitting panel such as an organic EL panel. Specifically, this figure shows exemplary variations among the I-B characteristics of the R, G, and B element groups. As shown in the figure, the R, G, and B element groups each exhibit a different current value I at the same brightness. FIG. 1B shows V-I characteristics of the self-emitting panel. Specifically, this figure shows exemplary variations among the V-I characteristics of the R, G, and B element groups. As shown in the figure, the R, G, and B element groups each exhibit a different voltage level V at the same control current I.
In view of the above problem that there are variations among the characteristics (I-B characteristics and V-I characteristics) of the. R, G, and B self-emitting element groups, it is an object of the present invention to provide a self-emitting display driving circuit capable of adjusting the gamma characteristics of the R, G, and B element groups separately such that each group exhibits substantially the same brightness characteristic.